Oven regulated permanent magnet having thermal lagging between the oven and the magnet



Oct. 21, 1969 I R. a. ems 3,474,222

OVEN .REGULATED PERMANENT MAGNET HAVING THERMAL LAGGING BETWEEN THE OVENAND THE MAGNET Filed June 12. 1967 TEMPERATURE CONTROL l8 FIG.I

INVENTOR.

- ROBERT E. GANG v BY AMBIENT ROOM W TEMPERATURE ATTORNEY United StatesPatent U.S. Cl. 219210 6 Claims ABSTRACT OF THE DISCLOSURE A permanentmagnet apparatus which is temperature regulated to be especially suitedfor high resolution gyromagnetic resonance spectroscopy. The magnetapparatus includes a pair of permanent magnets axially extending towardeach other to define a magnetic gap between their adjacent ends. Thepermanent magnets are enclosed by a yoke structure to magneticallyshield the gap. A thermostated oven envelops the magnet and yokestructures for holding them at a predetermined temperature above ambientroom temperature. A blanket of thermally insulative foam materialenclosed the magnet and yoke structures physically supporting same fromthe oven. The foam serves to thermally lag the magnet relative to theoven such that the time rate of change of the magnet is much less thanthat of the oven. Also, the oven is thermally lagged by a similarblanket of insulative foam relative to ambient room temperature. Threeseparate concentric magnetic shields serve to add additional magneticshielding for the gap.

Description of the prior art Heretofore, temperature regulated permanentmagnets have been used for high resolution nuclear resonance. One suchmagnet employed a thermostated oven which enclosed the magnet forholding same at a predetermined temperature above the ambient roomtemperature. The oven was thermally lagged by insulative foam from theambient room temperature environment. However, the magnet was notthermally lagged with respect to the oven. Actually, hot air, which washeated by the oven, was circulated around the magnet inside the oven forcausing the magnet to be essentially in good thermal contact with theoven. The problem with this arrangement is that the magnet temperaturetends to follow relatively short term temperature changes of the oven.The time rate of change of temperature of the permanent magnet is veryimportant for high resolution gyromagnetic resonance spectroscopy sincethe field intensity changes radically with changes in magnettemperature. Field changes produce unwanted drifts in the resonancespectra. Such drifts can be corrected by relatively complex andexpensive field-frequency control circuitry but it is desired to reducesuch temperature dependent drifts by less expensive means.

Summary of the present invention The principal object of the presentinvention is the provision of an improved permanent magnet apparatus.

One feature of the present invention is the provision, in a permanentmagnet system temperature regulated by a thermostated oven, of a thermallagging enclosure between the oven and the magnet, whereby the magneticfield produced by the magnet is stabilized against temperaturefluctuations of the oven.

Another feature of the present invention is the same as the precedingfeature wherein the thermal lagging between the oven and the magnetcomprises an insulative foam structure enclosing the magnet and whichalso serves to physically support the magnet within the oven structure,whereby the magnet support structure is simplified.

Another feature of the present invention is the same as any one or moreof the preceding features wherein a magnetic shield encloses the magnetand yoke structure and is disposed adjacent the inside surface of theoven for holding the shield at a uniform temperature.

Another feature of the present invention is the provision of a thermallyinsulative foam structure enclosing the oven for thermally lagging theoven relative to ambient room temperature.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

Brief description of the drawings FIG. 1 is a longitudinal sectionalview of a magnet apparatus incorporating features of the presentinvention,

FIG. 2 is a schematic electrical equivalent circuit for the temperaturecontrol system employed with the magnet apparatus of FIG. 1, and

FIG. 3 is a plot of temperature change in degrees centigrade versus timein hours for various dilferent magnet systems.

Description of the preferred embodiments Referring now to FIG. 1, thereis shown a temperature regulated magnet system incorporating features ofthe present invention. The magnet 1 includes a pair of axially extendingpermanent magnets 2 and 3 extending toward each 'other to define amagnetic gap 4 between their adjacent ends. The permanent magnets 2 and3 are supported at their far ends from the ends of a generallyegg-shaped magnetic yoke structure 5 which encloses the permanentmagnets 2 and 3 and serves as a magnetic shield for shielding the gap 4from external magnetic effects. A pair of magnetizing coils 6 and 7 arewound on coil forms 8 and 9 also carried from the ends of the yoke 5. Inaddition, a heater coil may be wound in with the magnetizing coils 6 and7 for initially heating the magnet 1 to its operating temperature. Asequence of high current pulses are fed through the magnetizing coils 6and 7 for initially magnetizing the permanent magnets 2 and 3.

In a typical example, the magnets 2 and 3 are each formed by a stack ofthree disks of permanent magnet material such as Alnico V, each diskbeing 6" in diameter and 2.5 thick. The yoke 5 is, for example, a oneinch thick shell of soft iron to provide a low reluctance flux returnpath around the magnets 2 and 3. The magnets 2 and 3, when fullyenergized, produce a field intensity of about 14.1 kg. in a 0.5" gap 4which is approximately 3" in diameter. The magnet 1 is temperaturecompensated by Carpenters steel shunting sleeves coaxially disposedaround each of the magnets 2 and 3, as described and claimed incopending U.S. patent applications 512,422, and 512,423, filed Dec. 8,1965 and assigned to the same assignee as the present invention. Astemperature compensated, field intensity of the magnet 1 has atemperature coefficient of about 30 parts per million per degree C.

A hollow cylindrical oven structure 11, closed on its ends as of type1100 aluminum having a wall thickness of 0.125", encloses the magnet 1.Non-inductive printed circuit heating elements 10 are formed on theoutside surface of the oven 11 for heating the oven 11 to apredetermined temperature such as 36:0.001" C., which is well aboveambient room temperature. A hollow cylindrical mu-metal magnetic shield12, as of 0.050" wall thickness, is disposed adjacent the inside surfaceof the oven 11. The mag. netic shield 12 serves to additionally shieldthe gap 4 from external magnetic disturbances and effects. A thermallyinsulative foam structure 13 fills the space between the magnetic shield12 and the enclosing magnetic yoke for thermally logging the magnet 1relative to the oven 11. In a typical example, the foam 13 is Freonblown polyurethane. The enclosing foam structure 13 also serves tosup-port the weight of the magnet 1 from the Oven 11. In a typicalexample, the foam 13 has a minimum thickness of 1 inch.

A second hollow cylindrical mu-metal magnetic shield 14 encloses theoven 11 for adding additional magnetic shielding for the gap 4. In atypical example, the shield 14 has a wall thickness as of 0.030". Asecond thermally insulative foam structure 15 fills the space betweenthe second magnetic shield 14 and the oven 11. The second foam structure15 serves to support the oven 11 from the second shield 14, is made ofthe same material as the inner foam structure 13, and has a thickness ofabout 1 inch.

A third hollow cylindrical magnetic shield 16, as of soft iron having awall thickness of 0.090", encloses the second magnetic shield 14. Thethird shield 16 serves to shield the gap 4 from external magneticeffects. A third thermally insulative foam structure 17, made of theaforecited foam material, as of 1 inch thick, fills the space betweenthe outer shield 16 and the second magnetic shield 14. The third foamenclosure 17 serves to support the second shield 14 from the outershield 16. The second and third foam enclosures, 15 and 17 respectively,provide thermal lagging for the oven 11 relative to the surroundingambient room temperature environment. The outer shield 16 is supportedwithin a cradle contained within a suitable cabinet, not shown. A bore18, as of 2" in diameter, passes radially through the various shieldingenclosures and magnetic yoke 5 to provide access to the magnet gap 4.When the magnet is used with a gyromagnetic resonance spectrometer thesample probe, not shown, is inserted into the gap 4 through the accessbore 18.

Referring now to FIG. 2, there is shown an electrical equivalent circuitfor the thermal system of the magnet and its surrounding oven andshields. In the equivalent circuit, voltage is equivalent totemperature, current is equivalent to heat, resistance is equivalent tothermal insulation, and capacity is equivalent to heat capacity. The

thermostat circuit includes an electrical bridge 22 for sensing thedifference between the temperature of the oven 11 and a referencetemperature. The reference temperature is set by a reference resistor Rplaced in one arm of the bridge. The reference resistor R has a preciseresistance to parts per million per degree centigrade. The temperatureof the oven 11 is sensed by a resistive thermistor network 23 formed byfour thermistors R R R and R connected in two parallel branches. Thethermistors are afiixed to the outside of the oven 11 at widely spacedpoints and connected such that temperature gradients are cancelled outto obtain an average temperature reading for the oven 11. The bridge isexcited by two reference signal generators 24 and 25. The signalgenerators produce square wave outputs which are 180 out of phaserelative to ground such that there will be no output signal at terminal26 when the resistance of the thermistor network 23 is exactly equal tothe resistance of reference resistor R. However, when the temperature ofthe oven 11 departs from the predetermined temperature, this will shiftthe resistance of the thermistor network and produce an output signal atterminal 26. The output of the bridge 22 is fed to a power amplifier 27and thence to the heating elements on the oven 11 for heating samesufiiciently to balance the bridge 22.

In a typical example, the oven 11 has a heat capacity of 1 C. per 2watt-hours, the foam insulators 15 and 17 for the oven have aninsulation of 1 watt/ C., the thermal insulation 13 for the magnet 1 hasan insulation of 1 Watt/ C., and the magnet 1 has a heat capacity of 1C. per 25 watt hours. The amplifier has a gain of 300.

Referring now to FIG. 3, the operation of temperature control system forthe magnet 1 will be described. It is assumed for the sake ofexplanation that the ambient room temperature is suddenly increased by 1C. in the manner of a step function increase. The oven 11 with itsexternal thermal lagging has a time constant of about 2 hours. Thesensor 23 will sense the change in temperature and correct thetemperature of the oven 11 as indicated by line 31 such that the totaltemperature rise of the oven 11 is only C. However, the thermostatchanges the temperature of the oven 11 with an initial time rate ofchange equal to the slope of 1 C./2 hours, as indicated by line 32.However, due to the thermal lagging of the internal foam structure 13and due to the large heat capacity of the magnet 1, the time constant ofthe magnet 1 is 25 hours. Thus, the magnet 1 changes its temperature byA C., as shown by curve 33, with an initial maximum time rate of changein temperature of C./ 25 hours corresponding to line 34. During a sixminute period, the magnet 1 changes temperature less than C. for a 1 C.step change in the ambient.

If it were not for the internal thermal lagging produced by insulativestructure 13 and if the magnet 1 was thermally connected to the oven 11,as in the prior art circulating air scheme, the magnet temperature wouldfollow curve 35 and would have an initial time rate of change of 1 C./25 hours, as indicated by line 36. Thus, the thermal lagging 13 reducesthe time rate of change of the magnet 1 by the gain of the amplifier 27.In the example cited, this is a factor of 300 reduction as compared tothe prior art scheme.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a permanent magnet apparatus, means forming a pair of permanentmagnets axially extending toward each other to define a magnetic gap inthe space between their adjacent ends, means forming a magnetic yokestructure interconnecting said pair of magnets to provide a flux returnpath around said pair of magnets, means forming an oven structure havinga thermally conductive metallic wall enclosing said permanent magnetsand said yoke structure, means including a heating element disposedexternally of said metallic wall of said oven in heat exchangingrelation therewith for heating and controlling the temperature of saidoven to a predetermined temperature substantially above ambient roomtemperature, the improvement comprising; means forming a thermallyinsulative structure substantially completely enclosing said permanentmagnet and yoke structures, said thermally insulative structure beingdisposed between and substantially filling the space between saidthermally conductive Wall of said oven and said magnet and yokestructures for thermally lagging said magnet and yoke structuresrelative to said enclosing oven structure, an access opening throughsaid oven Wall and communicating with the magnetic gap of the magnet.

2. The apparatus of claim 1 wherein said yoke structure substantiallyencloses said pair of permanent magnets and the gap therebetween formagnetically shielding said magnets and gap from external magneticeffects, and wherein said enclosing thermally insulative structureincludes a thermally insulative foam structure serving tophysicallysupport said magnet and yoke structures within said ovenstructure.

3. The apparatus of claim 1 including means forming a magnetic shieldstructure enclosing said magnet and yoke structures and disposed insideof said oven structure for additionally shielding said gap from externalmagnetic eifects.

4. The apparatus of claim 1 including a second thermally insulativestructure enclosing said oven structure for thermally lagging said ovenrelative to changes in the temperature of the ambient room temperature.

-5. The apparatus of claim 4 wherein said second thermally insulativestructure includes a thermally insulative foam structure serving tophysically support said oven therewithin.

6. The apparatus of claim 4 including means forming a second magneticshield structure enclosing said second thermally insulative structurefor additionally shielding the magnetic gap from external magneticeifects.

6 References Cited UNITED STATES PATENTS 1,884,797 10/1932 Meyer 219-2103,046,473 7/1962 Kessler et a1. 3245 3,273,634 9/1966 Snelling 165-1FOREIGN PATENTS 1,321,792 2/ 1963 France.

10 JOSEPH V. TRUI-IE, Primary Examiner C. L. ALBRITTON, AssistantExaminer U.S. Cl. X.R.

